1. Field of the Invention
The present invention relates to sliding means that have been extensively used in machines and instruments as diverse as semiconductor manufacturing apparatus, machine tools, industrial robots, conveyors and others. It is more particularly concerned with a sliding means with built-in moving-magnet linear motor, in which an exciting winding is arranged in a stationary bed while a magnet is installed in a moving table.
2. Description of the Prior Art
In recent years, multi-axis stages and moving mechanisms employed in the diverse technical fields as described above have required more and more sliding means, which are compact or slim in construction and light in weight, and moreover able to operate with high propulsion, high speed and high response to provide high speed travel and accurate position control for works, tools, articles and instruments. Linear motors commonly used in the sliding means involve two broad types. The first, called moving-coil linear motor, has a stator of field magnet mounted on a stationary bed, and moving-armature coils arranged on a table movable lengthwise of the bed in space one after another such that they lie a preselected phase angle. The second, called moving-magnet linear motor, has a stator of armature windings arranged lengthwise over the entire length of a bed, and a moving-field magnet of permanent magnet arranged on a table movable in a sliding manner along the length of the bed.
Japanese Patent Laid-Open No. 322232/1996 discloses a linear motor installed in a knitting machine to drive a knitting needle in reciprocating motion. The liner motor is comprised of a plurality of built-in moving-coil liner motor units each of which has a moving assembly composed of a backing plate made therein a window, a resilient sheet member fixed on any one side of the backing plate with adhesive, and exciting windings, for example three windings, arranged on any one surface of the sheet member in a manner to be partly accommodated in the window. The exciting winding is made in the form of flat ellipse where the axial direction of the winding extends thickness-wise of the linear motor unit. The moving assembly is arranged for linear movement between stator assemblies confronting one another, each of which is composed of a backing plate made of ferromagnetic material such as steel, and a plurality of permanent magnet, for example six pieces arranged on the backing plate in juxtaposition along the traveling direction of the moving assembly. The construction in which the exciting windings are accommodated in the associated window in the backing plate reduces the overall thickness or height of the moving assembly. Linear displacement-measuring means is composed of a linear scale extending along the moving direction of the moving assembly, and a sensor head installed on any one of the confronting stator assemblies.
A moving-magnet brushless dc linear motor is disclosed in Japanese Patent Laid-Open No. 298946/1989, in which a semiconductor rectifier is arranged for each coil, and two sets of three-phase coil groups are arranged to provide a linear motor of three-phase conduction system.
A sliding means adapted to be used for machine tools and industrial robots is disclosed in Japanese Patent Laid-Open No. 266659/1997, which is a senior application of the present applicant. The prior sliding means includes a driving source made of an electromagnetic linear actuator and a built-in moving-magnet uniaxial linear motor to control with precision a position of a driven article. With the prior sliding means cited just above, an electromagnetic linear actuator is arranged between a moving table and stationary bed of steel or magnetic material and at least any one of the table and the bed is constructed to serve a part of magnetic circuit of the electromagnetic linear actuator, concretely the function of either magnet yoke or coil yoke. The prior sliding means has no need of providing separately yokes for establishing magnetic circuit, which might make the sliding means bulky, thus reduced in the number of parts required, and made inexpensive in production cost and slim in construction.
The sliding means disclosed in the above Japanese Patent Laid-Open No. 266659/1997 will be explained below, with referring to FIGS. 14 and 15. A sliding means 51 with an built-in linear motor is composed of a stationary bed 52 and the moving table 53, both of which are made of magnetic material such as steel to serve the function of magnetic circuit, or magnet yoke and coil yoke, thereby rendering the linear motor small or compact in size. The sliding means 51 with built-in linear motor has the stationary elongated bed 52, and the moving table 53 mounted on the bed 52 for linearly reciprocating movement lengthwise of the bed 52 by virtue of linear motion guide units 54. The linear motion guide units 54 are comprised of two track rails 55 arranged on the bed 52 in parallel with each other, and four sliders 56 fitting over the associated track rail 55 for sliding movement. In the linear motion guide units 54, load raceway areas are provided between confronting raceway grooves, one of which is formed on lengthwise sides of the track rails 55 while the counterpart is formed on the sliders 56. The sliders 56 are allowed to move with smooth along the track rails 55 as rolling elements run through the load raceway areas. The table 53 is bored with holes 58 through which screws fit to fix a work on the table 53. An end block 61 and a connector block 62 are secured to the lengthwise opposing ends of the bed 52, each to each end, with fixing bolts 63, 64 to define a tolerable range of operating stroke of the table 53. The bed 52 is made with holes 65 through which bolts 66 fit to anchor the bed 52 to a platform.
An armature 70, which is a primary side of the sliding means 51, is comprised of a coil board 71 and eight pieces of armature windings 72 arranged on the underside of the coil board 71 in juxtaposition along the moving direction of the table 53. The bed 52 is recessed lengthwise at 73 on the upper surface thereof, where the armature 70 is accommodated through an insulating film 74. Hall-effect elements 75 are arranged on the coil board 71 in conjunction with the armature windings 72, each to each winding. The Hall-effect elements 75 are to issue a signal in response to an amount of magnetic flux created by a secondary field magnet 90, which is detected when the field magnet 90 approaches the Hall-effect elements 75. Excitation of the armature windings 72 is controlled depending on the signal issued out of the Hall-effect elements 75. The armature 70 is jointed to the bed 52 by means of machine screws 76 fitting through spacers 77, which make abutment at their opposing ends against both the bed 52 and the coil board 71 at locations offset widthwise of the bed 52 between any two adjacent armature windings 76 from one another.
The bed 52 is also made with a recess 79 at the underside opposite to the upper recess 73. A driving board 80 is received in the lower recess 79 through an insulating film 81. The driving board 80 is to apply electricity to the armature windings 72, and mounted with a driving circuit 82 composed of diverse electronic components. The driving board 80 is connected with the coil board 71 via connectors 83, 84 extending through a hole 85 bored vertically through the bed 52. In addition, the lower recess 79 in the bed 52 is closed with a cover 86.
The field magnet 90, which is the secondary side of the linear motor, is installed in a recess 92 formed in the table 53 and secured to the underside of the table 53. The field magnet 90 is composed of platy magnets 91 arranged such that unlike poles (N, S) on the platy magnets 91 alternate along the moving direction of the table. The table 53 mounted with the platy magnets 91 provides a magnet yoke forming a part of magnetic circuit, while the bed 52 provides a coil yoke for each armature winding 72, which also forms a part of magnetic circuit. When the preselected current is applied to each armature coil 72, a thrust force is created between the primary and secondary sides on the basis of Fleming's rule to drive the table 53 integral with the secondary field magnet 90 in a sliding manner by virtue of the linear motion guide units 54.
To determine the reference position of the table 53 with respect to the bed 52, a Hall-effect element 97 is installed inside the second armature winding 72 from the left. The reference position may be identified by a signal issued at a time when the Hall-effect element 97 has detected the leftmost platy magnet 91 in the field magnet 90. Besides, two Hall-effect elements 98, 99 are attached to the coil board 71 inside the leftmost and rightmost armature windings 72, each to each winding, to provide limit sensors that ensure keeping the table 53 from travelling over the tolerated range of moving stroke. Each Hall-effect element 98, 99, when the table 53 has traveled over the tolerated range of the operating stroke, may respond to any associated pole at the leftmost and rightmost extremities of the field magnet 90 to issue a signal reporting the accidental event where the table has run away from the desired stroke. In order to monitor the relative location of the table 53 to the bed 52 in the sliding means 51, the table 53 is provided at one lengthwise side thereof with a magnetic linear scale 95 in which unlike magnetic poles (N, S) are arranged alternately with a fine pitch along the moving direction of the table 53, while the bed 52 has a sensor head 96 responsive to the magnetic scale 95.
In the sliding means 51 with built-in linear motor constructed as stated earlier, there is employed a system in which electric conduction is controlled every each armature winding 72 and, therefore, both the driving board 80 and the driving circuit 82 are built in underneath the bed 52. This system makes the sliding means complicated and bulky in construction. Besides, the linear scale is made of magnetic scale.
In a sliding means with built-in moving-magnet linear motor in which a table is arranged on a bed for sliding movement, the bed having supported thereon an armature winding while the table being mounted with a field magnet on a surface confronting the bed so that the current flowing through the armature winding interacts in an electromagnetic manner with magnetic flux created by the field magnet to drive the field magnet together with the table, it has been desired to make the sliding means light in operation, simple and slim in construction, light in weight and much more precious in position control of the table to the bed. To this end, there are problems to be solved in conduction system for the armature winding, material for the field magnet, design of the high resolving-power encoder and fixing means for the sensor cords.